An impedance/conductivity cavity probe can be formed using a rectangular waveguide. A closed rectangular cavity has six metal walls with a small iris located in the center of one of the walls. Electromagnetic energy (e.g., radio frequency (RF) or microwave energy) may be coupled into and out of the cavity via the iris. The iris will couple energy at a particular frequency (e.g., a resonant frequency) into the cavity while not coupling energy at frequencies other than the particular frequency. Thus, as energy is fed to the impedance/conductivity cavity probe over a band of frequencies, most of the energy will be returned except at the resonant frequency. If one plots the returned energy as a function of frequency, a sharp null will be located at the resonant frequency.
An open-ended cavity probe may be formed by removing one of the six walls of the closed rectangular cavity, resulting in a cavity with five walls (e.g., an open-ended cavity). The open-ended cavity probe may be used to measure properties of a sample by placing the sample proximate to the open end of the open-ended cavity, thereby effectively closing the open-ended cavity.
In some examples, the sample may be a conductive material. Closing the open-ended cavity probe with the conductive material sample may result in a null at the resonant frequency of the chamber including one wall corresponding to the sample. For a sample with conductivity close to that of a metallic surface, the null may have the same depth as if the original metal wall (e.g., of the closed rectangular cavity) was still in place. If the material is less conductive than a metallic surface, then the null may still be located at the resonant frequency, but it may not be as deep. As the material becomes less conductive, the null may become shallower until it completely disappears.
The open-ended cavity probe can be used to measure the conductivity of a given sample based on a relationship between the null depth and conductivity. The open-ended cavity probe may be calibrated by measuring probe responses to conductive material samples, referred to as materials standards, and establishing a relationship between the probe responses (e.g., null depth of the probe responses) and the known constitutive properties of the materials standards (e.g., conductivity).
In other examples, the sample may include a conductive material coated by a non-conductive (e.g. dielectric) layer. The open-ended cavity probe may still be used to measure conductivity/impedance of samples that include a thin dielectric topcoat, although a different calibration/relationship needs to be established than when the samples do not include a dielectric topcoat. For example, the null depth returned by the open-ended cavity probe when closed by a sample including a conductive material coated by a non-conductive layer may still be related to the conductivity of the sample (as with a purely conductive material), but the relationship between null depth and conductivity may be modified somewhat by the presence of the dielectric topcoat. In addition, the null measured when the open-ended cavity probe is closed by a sample including a conductive material coated by a non-conductive layer may no longer present at the same resonant frequency as when the sample is a purely conductive material. The amount of shift may be influenced by the thickness of the dielectric topcoat. A cavity probe is typically calibrated to measure conductivity/impedance of samples that include a dielectric topcoat using different materials standards (e.g., samples) formed of different conductive materials in combination with two thickness of a dielectric for the purposes of performing the calibration. However, this can be problematic in a field or mobile environment as the samples can get damaged or lost. In addition, the materials of the samples can degrade with time, and it may be problematic to make materials consistently.